Determination of the crystal structure of magnesium perchlorate hydrates by X-ray powder diffraction and the charge-flipping method

摘要

X-ray powder diffraction (XRD) data were used to solve the crystal structures of phases in the magnesium perchlorate hydrate system, Mg(ClO _4)_2·nH_2O (n = 4, 2). A heating stage and humidity generator interfaced to an environmental cell enabled in-situ XRD analyses of dehydration reactions under controlled temperatures and partial pressures of H_2O (P_(H2O)). The crystal structures were determined using an ab initio charge-flipping method and were refined using fundamental-parameter Rietveld methods. Dehydration of magnesium perchlorate hexahydrate to tetrahydrate (348 K) results in a decrease in symmetry (space group = C2), where isolated Mg~(2+) cations are equatorially coordinated by four H_2O molecules with two [ClO_4]~- tetrahedra at the apices. Further dehydration to the dihydrate (423 K) leads to bridging of the isolated packets to form double corner-sharing chains of octahedra and polyhedra (space group = C2/m).